I also just finished 1L, and I don't attend Queen's, so take this with a grain of salt.
I'm not sure if your curve is different than ours, but from my understanding, your grades are actually pretty good (probably a B+ average) - which at least at Osgoode indicates you are in the top-third of your class.
It also seems like you know exactly where your areas for improvement are - i.e. time management (both during exams and during the school year to ensure you don't fall behind), and writing organization. When you are able to organize your time well on an exam, you'll be less likely to miss issues or make mistakes that you wouldn't otherwise make.
Asking for feedback is a great first step. As for next year: start reviewing earlier, keep up with your readings, write practice exams, and ask questions to clarify areas of uncertainty. If you studied with a group this year and you didn't find that helpful, consider trying a new approach for the coming year. I think the organization piece to make sure you aren't crammed going into exams will be the most helpful though.

I didn't go personally because I had a prior commitment, but it is definitely worth it - I've only heard excellent things about it. Missing class for something like this is OK, most lectures are recorded making it easy to catch up. Would recommend

Osgoode has a part-time ("Extended Time") program, though I caution you it's not part time in the sense that you have evening classes that you attend after working for the day. You follow the same course schedule as other students, except you take fewer courses. I do know that you can pick and choose courses from different sections to make things more convenient though (e.g. trying to have all your classes on 1-2 days).
I believe the Extended-Time program is for people with extenuating circumstances (e.g. childcare responsibilities), but you can certainly contact the school and ask.

They're not a particularly good use of time in terms of learning about the firm in any meaningful way. I think the one potential use (that I've seen my friends leveraging them for) is to connect with lawyers and then follow up to talk to them about their practices.
That being said, I personally think the value of this kind of networking lies beyond the formal recruitment cycles. I'd be fairly skeptical to think that attendance at a firm tour would give someone a meaningful advantage at obtaining an OCI over someone who did not attend, unless they were identical in all other regards.

The word limit is quite short, approximately 150 words/2 questions from what I remember. The questions are just that - generic. Think along the lines of "why do you want to volunteer here" and "what relevant experience do you have"

Like you said, since there aren't many easily comparable metrics between candidates other than grades, they're generally used to infer that a student is intelligent, works hard, and is able to manage their time reasonably well (according to the student committee at my firm).
That being said, students with average grades are also capable of being intelligent and hardworking, just as students with above-average grades are capable of being poor employees (though I would say this tends to be an outlier). I'd imagine this is because grades merely get you in the door, whereas the rest of the interview/recruitment process is where these "soft" factors are assessed.
At any rate, 1L "top-ish" student here, and I make mistakes on a weekly basis. More often than not these are flagged by the lawyer who assigned the work, and I'm given the opportunity to address/learn from them.

This is a bit of a late reply, but I definitely struggled with anxiety and depression before coming into law school, and was able to access supports available at my school and externally that were invaluable in my being successful in law school (both in terms of academics, and quality of life).
There are a few resources I'd like to point out. The first is the Law Society's Member Assistance Program that is also available for law students to access. Second, OSAP has a bursary for students with disabilities (including mental health) that you can access if you're eligible, and this can help pay for counselling or other supports if you need them. There will also be school-specific supports that are offered. I can't speak to all law schools, but you're welcome to PM me if you have questions about my experience or want to chat further.

I heard that they set up three interview dates for first interviews - this past Friday, yesterday, and today. Unsure if it was just for that particular position or for all of them, but I believe this was for tax.

Personally am between Blakes and Dentons! This is based on a combination of practice areas I'm interested in, conversations with previous students, and visiting the firms. Things may sway in the course of interviews though and I'm open to that.